A lithium secondary battery has many advantages such as relatively high energy density, high operation voltage, excellent preservation and long life span in comparison to other secondary batteries, and thus the lithium secondary battery is widely used for various portable electronic devices such as personal computers, camcorders, cellular phones, CD players, PDA or the like.
Generally, a lithium secondary battery includes a positive electrode and a negative electrode disposed with an electrolyte being interposed between them. Here, the positive electrode is configured so that a positive electrode active material is attached to a positive electrode current collector, and the negative electrode is configured so that a negative electrode active material is attached to a negative electrode current collector.
In the lithium secondary battery, the negative electrode current collector is generally made of an electrolytic copper foil. In this case, if equipment conditions are not controlled strictly, many machine direction (MD) buckles occur.
The MD buckles mean a defect of an uneven shape, generated in a MD direction appearing at a copper foil rolled after a foil preparing process.
The MD buckles may cause take-back from customers when products are delivered, and may also cause coating deviation and active material separation when an active material is coated.
It is known that among causes of MD buckles which have been revealed so far, material causes have a relation with a weight deviation. However, in case of the copper foil, even though the weight deviation is strictly controlled, MD buckles are greatly generated, and thus a scheme to solve this problem is demanded.